(a) Field of the Invention
The present invention relates to a deflection yoke suitable for use, for example, in a video display or a video game, and especially to a means for effectively reducing electromagnetic interference radiation produced from a deflection yoke.
(b) Description of the Related Art
Sources of electromagnetic interference radiation in a video display or the like include a deflection yoke, a flyback transformer and other coils. Of these, the primary source is the deflection yoke.
The electromagnetic interference radiation from a deflection yoke is a magnetic field induced by a current of sawtooth waveform which flows through each horizontal deflection coil during each horizontal retrace interval. Various methods have been proposed with a view toward preventing the occurrence of electromagnetic interference radiation from a deflection yoke. One example is disclosed in Japanese Patent Laid-Open No. 82633/1987.
The invention described in the above patent publication is concerned with the elimination of electromagnetic interference radiation which is produced from the fringe of the front end section (end portion on the screen side of a cathode ray tube) and from the fringe of the rear end section (end section on the neck side of the cathode-ray tube) of each saddle-shaped horizontal deflection coil. Described specifically, the invention of the above patent publication features the use of cancellation coils having a substantially similar saddle-like shape to the horizontal deflection coils but only 3-10% of the number of turns of the horizontal deflection coils. The cancellation coils are connected in series with the corresponding horizontal deflection coils, so that magnetic fields, namely, those electromagnetic interference radiations produced from the fringes of the front and rear end sections of each horizontal deflection coil, may be canceled by magnetic fields produced from the fringes of the front and rear end sections of the corresponding cancellation coil.
The above conventional technique can eliminate electromagnetic interference radiation produced from both the front and rear fringes of the horizontal deflection coil, but electromagnetic interference radiation also occurs from the front and rear end sections of each horizontal deflection coil other than the fringes. In front of the cathode-ray tube, electromagnetic interference radiation from the front end section of each horizontal deflection coil other than the corresponding front end fringe is of primary concern rather than that produced from the front end fringe.
The above-described conventional method can reduce electromagnetic interference radiation from each front end section to a certain extent. However, any attempt to reduce the radiation further however results in more electromagnetic interference radiation from each rear end section.
Production of electromagnetic interference radiation from the deflection yoke will hereinafter be described with reference to FIG. 9 which appears next to FIG. 3.
Horizontal deflection coils 1,1' and vertical deflection coils 2,2' are provided in association with an unillustrated cathode-ray tube. When a deflection current of sawtooth waveform is caused to flow through the horizontal deflection coils 1,1', a magnetic field (hereinafter called the "primary magnetic field") 6 is projected ahead from front end sections (end sections on the screen side of the cathode-ray tube) of the horizontal deflection coils 1,1' as indicated by solid arrow marks. In addition, another magnetic field (hereinafter called the "front end fringe magnetic field") 5 is also projected ahead from fringes (hereinafter called the "front end fringes") of front end sections of the horizontal deflection coils 1,1' as indicated by dashed arrow marks. The front end fringe magnetic field 5 is a backing magnetic field relative to the primary magnetic field 6.
On the neck side of the unillustrated cathode ray tube, a further magnetic field (hereinafter called the "secondary magnetic field") 9 is also projected behind from rear end sections of the horizontal deflection coils 1,1' as indicated by solid arrow marks. In addition, a still further magnetic field (hereinafter called the "rear end fringe magnetic field") 10 is also projected behind from fringes (hereinafter called the "rear end fringes") 8,8' of the rear end sections of the horizontal deflection coils 1,1' as indicated by solid arrow marks.
These primary magnetic field 6, front end fringe magnetic field 5, secondary magnetic field 9 and rear end fringe magnetic field 10 are all electromagnetic interference radiations. Since the horizontal deflection current varies considerably during each horizontal retrace interval, these electromagnetic interference radiations occur at substantially high intensities. In particular, the primary magnetic field 6 and the rear end fringe magnetic field 10 extend to substantial distances on the screen side and neck side of the cathode-ray tube, respectively.
Regarding electromagnetic interference radiation, how much the intensity of magnetic field would vary depending on the deflection current during each horizontal retrace interval is measured at a position remote over a predetermined distance from the cathode-ray tube. Each measurement value is expressed in terms of mT/sec. It has been recognized that the primary magnetic field 6 is dominant as electromagnetic interference radiation when measured at a point A remote over a predetermined distance from the screen of the cathode-ray tube in FIG. 9 and the rear end fringe magnetic field 10 is dominant as electromagnetic interference radiation when measured at a point B remote over a predetermined distance from the rear end of the cathode-ray tube on the other hand.
As described above, the magnetic field produced from the front end sections of the horizontal deflection coils 1,1' are dominant as electromagnetic interference radiation on the front side of the deflection yoke but the magnetic field produced from the rear end fringes of the horizontal deflection coils 1,1' is dominant as electromagnetic interference radiation on the rear side of the deflection yoke. At both the front end sections and the rear end sections of the horizontal deflection coils 1,1', the magnetic fields produced there are opposite in direction to the magnetic fields produced from the corresponding fringes.
It is therefore impossible to eliminate electromagnetic interference radiations produced from both the front end section and the rear end section of the horizontal deflection coils 1,1' by the above-described conventional technique which is intended to eliminate electromagnetic interference radiations produced from the fringes of both the front end section and the rear end section of the horizontal deflection coils 1,1'.
The cancellation coils described above are mounted near the front end section of the deflection yoke symmetrically relative to the central axis of the deflection yoke. When the deflection yoke is attached to a cathoderay tube and there is no external magnetic body, e.g., no magnetic shield plate around the cathode-ray tube, the magnetic fluxes produced from the cancellation coils become symmetrical so that the electromagnetic interference radiation can be suppressed around the entire periphery of the cathode-ray tube.
In an actual video display, the outer periphery of a chassis is however often covered by a magnetic shield plate to reduce electromagnetic interference radiation from components other than a deflection yoke. When the deflection yoke equipped with cancellation coils and attached to a cathode-ray tube is installed within the video display and the intensity of electromagnetic interference radiation is measured, the intensity of electromagnetic interference radiation is found to be higher at certain locations due to the arrangement of the magnetic shield plate. There is hence the potential danger that deleterious effects may be given to the human body.